Most of the present automotive bodies are configured with monocoque bodies, each of which is formed by integrating a frame and a body that typically constitute a frame body, and the lower part of the body has a structure called a platform. FIG. 19 is an explanatory view schematically showing a structure of a platform 102 of an automotive body 101 in a simplified form.
As shown in this drawing, the platform 102 of the automotive body 101 is disposed in a part corresponding to the conventional frame. A dash panel 103 and a front floor panel 104 of the platform 102 are joined to each other with respective edge portions overlapped on each other, and the front floor panel 104 and a rear floor panel 106 are joined to each other with respective edge portions overlapped on each other.
A dash upper panel 103a and a dash lower panel 103b of the dash panel 103 are joined to each other with respective edge portions overlapped on each other. The dash panel 103 is a partition wall between an engine compartment in which an engine is placed and a cabin for vehicle occupants.
The front floor panel 104 has a floor tunnel 104a, upright flanges 104b disposed left and right, and plane portions 104c disposed left and right. The floor tunnel 104a provides a space for storing a propeller shaft and various kinds of piping in the center in the automotive width direction. The upright flanges 104b disposed left and right are joint portions for being joined to left and right side sills 105 having a closed section structure. The left and right plane portions 104c connects the floor tunnel 104a and the upright flanges 104b disposed left and right.
A rear floor front panel 106a and a rear floor rear panel 106b of the rear floor panel 106 are joined to each other with respective edge portions overlapped on each other.
The front floor panel 104 is required to have bending stiffness and torsional stiffness enough to suppress elastic deformation of a body due to a static load received from a mounted component such as a front seat and a load to the body from four tires while running. Furthermore, the front floor panel 104 is required to suppress occurrence of noise and vibration while running as much as possible so as to avoid uncomfortableness to the occupants, and to reduce weight to improve fuel efficiency of the automobile.
In order to meet such requirements, the art to obtain high stiffness and excellent noise and vibration characteristics without increasing the weight of a front floor panel by providing an convex-concave shape in a region that is to be the plane portion of the front floor panel is known.
For example, Patent Document 1 discloses the art of reducing uncomfortableness to occupants and increasing stiffness of the floor panel by increasing the resonance frequency of a floor panel, by forming convex portions on the floor panel, each of which is configured by combining the identical isosceles triangles so that a plane thereof has a particular angle.
Patent Document 2 discloses an insulator capable of securing sufficient stiffness without increasing the sheet thickness by forming a number of convex portions by embossing a heat insulator that is provided on a dash panel front surface or a lower part of a floor panel, with the convex portions having hexagonal shapes in plan view and the longitudinal sections which pass vertices forming a diagonal and are arc-shaped, and by arranging the convex portions so that flat sheet portions do not pass rectilinearly among the convex portions.
Patent Document 3 discloses the invention that reduces the sheet thickness of the floor panel while increasing stiffness with respect to the automobile width direction of the floor panel, by providing a bulged part that crossed with the floor tunnel and is oriented to the automobile width direction in the floor panel having the floor tunnel extended longitudinally in the central part of the automobile width direction.
The conventional technologies disclosed by Patent Documents 1 to 3 each aims at increasing stiffness, and noise and vibration characteristics by providing an convex-concave part in the entire front floor panel or the center of the plane portion. However, according to the research of the present inventors, the advantageous effect of increasing stiffness to such an extent that can reduce the sheet thickness of a floor panel cannot be obtained by using the technology disclosed in Patent Document 1, the manufacturing cost inevitably increases since a number of convex portions are formed in the sheet material by using the technology disclosed in Patent Document 2, and further, the technology disclosed in Patent Document 3 has the problem of an increase in stiffness anisotropy toward the end portions at the front and the rear of the floor panel.
Non Patent Document 1 and Patent Documents 4 to 9 do not necessarily relate to front floor panels, but disclose sheet materials or panels for automotive component members including convex-concave parts that can reduce stiffness anisotropy each by providing the identical shapes and areas for both top surfaces of convex-concave shapes that protrude in the up and down directions from the reference plane, and making the second moment of area large in every cross section.